WASHINGTON: Candidate Barack Obama offered a lofty vision of how his White House would operate. When the details of health reform were being hammered out, he vowed, ''We'll have the negotiations televised on C-SPAN so that people can see who is making arguments on behalf of their constituents, and who are making arguments on behalf of the drug companies or the insurance companies.''

The campaign even aired an ad singling out Billy Tauzin, the drug industry's chief lobbyist. ''The pharmaceutical industry wrote into the prescription drug plan that Medicare could not negotiate with drug companies,'' Obama said in the ad. ''And you know what? The chairman of the committee, who pushed the law through, went to work for the pharmaceutical industry making $2 million a year.'' Imagine that.

Now, it turns out, the Obama White House has cut a backroom — actually, Roosevelt Room — deal with Billy Tauzin: Drugmakers would ante up $80 billion in savings in return for a promise that Medicare wouldn't be allowed to negotiate drug prices.

''We were assured: 'We need somebody to come in first. If you come in first, you will have a rock-solid deal,' '' Tauzin told the New York Times.

Imagine that.

The White House, playing the political version of Deal or No Deal, is backing away, rather unconvincingly, from its initial confirmation. In New Hampshire on Tuesday, Obama raised the prospect of getting more from drug companies. But the episode underscores the dangerously wide gap between Obama's idealistic campaign-trail promises and the gritty realities of real-world governing.

Every new president encounters this tension, but it is, I think, more acute, and therefore more politically treacherous, for Obama. The fundamental allure of his candidacy was that he could not only wipe away the stains of the Bush administration but overcome the partisan jockeying and special interest influence-buying that have alienated voters from their government.

As Obama said in announcing his candidacy, ''In the face of a politics that's shut you out, that's told you to settle, that's divided us for too long, you believe we can be one people, reaching for what's possible, building that more perfect union.''

In reality, Obama's campaign did not always live up to Obama's rhetoric: He summoned voters to ''a politics that calls on our better angels'' but stooped to scare tactics when political need required them. As I wrote last October, ''Better angels, it seems, do not make the best campaign strategists.'' Or, I'd add, the best White House advisers.

In any event, the promises were so grand, the moment so inspiring, that the aftermath had to disappoint.

Seemingly simple campaign pledges turn out to be intractable problems. Closing Guantanamo and ending ''don't ask, don't tell'' are easier to proclaim than achieve. This is no surprise to anyone who took the time to wonder how, exactly, these would be accomplished. But campaigns worry about election first, implementation later.

Clear positions yield to political realities. Having Medicare negotiate prescription drug prices gives way to the need to get drug companies on board. The campaign climate-change plan to auction off all emissions permits morphs, without a presidential peep, into a House-passed measure that would hand out 85 percent of the permits as political candy to mollify lawmakers in districts that would be hard hit by strict emissions limits.

Especially in the arena of national security and foreign policy, the situation looks more complex from the vantage point of the Situation Room than it did from the campaign trail. It is easy to proclaim the need ''to restore our Constitution and the rule of law,'' harder to resist a continuation of Bush administration policies on issues such as the invocation of the ''state secrets'' privilege and the use of presidential signing statements.

It is easy to talk about a new era of engagement with Iran and North Korea, harder to figure out what to do when those regimes — murdering protesting citizens or conducting provocative missile launches — prove intransigent.

But the greatest peril for Obama, I think, lies in the question of whether he can produce the new, post-partisan, surmounting-special-interests politics that he envisioned during the campaign. In a month of raucous town-hall meetings and stalled legislation, that hardly seems likely. The story about the secret deal with Tauzin can only deepen the skepticism.

Which leads to the core question facing the still-young administration: What happens when people start to wonder whether they can really believe in this change?
Marcus is a Washington Post columnist. She can be marcusr@washpost.com.